Re: Toshiba TX3927 board boot problem.

[Ralf Baechle <ralf@oss.sgi.com>]

On Mon, Oct 29, 2001 at 04:17:23PM -0800, Alice Hennessy wrote:

> > This doesn't look right, you still need to enable the CU1 bit in the
> > status register to let the FP emulator kick-in.  FPU-less CPUs should
> > take a coprocessor unusable exception on any floating-point instructions.
> > I have been running this on several FPU-less CPUs, and it works fine for
> me.
> 
> Maybe the FPU-less CPUs you have been using define the CU1 bit as reserved
> or is unused (ignore on write, zero on read)? The TX3927 actually allows
> the setting of the CU1 bit.  Have you seen a case where you need to set
> the CU1 bit for the emulation to kick-in?   I would think that the CU1
> bit should never be set to one for FPU-less CPUs.

There are subtle differences in how CUx bits for unimplemented coprocessors
are handled in the various processors.  MIPS32 and MIPS64 specifies the
behaviour as 0 on read, writes ignored; previous processors such as the
R4000 handled this differently and as a consequence a fp instruction on
a fpu-less r4000 class cpu may either throw a CU or a reserved instruction
exception.  To make things easier for everybody this is documented in the
R10000 user's manual ...

  Ralf